Simulcast transmission is useful for increasing the coverage area in land-mobile radio systems as is well known to those skilled in the art. Simulcast transmission incorporates at least two transmitters simultaneously broadcasting identical information on the same frequency and being located to provide contiguous coverage over an area larger than would be possible for a single transmitter broadcasting alone. Precise time control of base band signals is required in simulcast transmission. For example, if base band signals are transmitted at incorrect times by the two transmitters, distortion occurs where signals are received from both transmitters with similar signal strengths. Such distortion is present when the various signals arrive at the receiving end with even slight phase or timing differences with respect to each other.
FIG. 1 shows a simplified simulcast transmission system 100 including two base stations or remote site transmitters 101 and 103. Remote site transmitter 101 provides coverage for an area 111, and subscriber units 105 and 107 receive transmitted messages 119 and 117, respectively, therein. Similarly, remote site transmitter 103 provides coverage for an area 113, and subscriber units 107 and 109 receive transmitted messages 118 and 121, respectively, therein. Coverage areas 111 and 113 overlap by coverage area 115, within which the subscriber unit 107 receives transmissions from both transmitters 101 and 103. Hence, the transmissions 117 and 118 are perceived as a single signal by the subscriber unit 107.
FIG. 2 depicts a new simulcast radio system 250 which is more fully described in patent application, "Simulcast Transmission System Having Predetermined Launch Times", filed Jan. 28, 1991, Ser. No. 646,577, and assigned to the assignee of the present Application, which is hereby incorporated by reference.
In the past, a typical transmission sequence began by sending a message signal 202 to a controller 204 for transmission to one or more coverage areas. The controller 204 distributes the message signal 202 to one or more remote site transmitters (e.g., 208, 210, etc.). Distribution was accomplished via an expensive microwave distribution system made up of a plurality of interconnect links (not shown). Each remote site transmitter 208, 210, etc., would transmit the message signal 202 upon receipt to a subscriber unit within the appropriate coverage area. The critical timing requirements necessitated the use of precisely calibrated or netted interconnect links (i.e., the total propagation delays are made identical across all interconnect links).
Simulcast transmission is improved by the transmission system 250 shown in FIG. 2. Here, the message signal 202 is received by the controller 204, as before, and further processed (e.g., converted to digital form) before sending the message signal 202 to the respective remote site transmitters 208, 210 via interconnect links 224, 226, respectively. Now, however, a precision timing reference signal receiver 219, 221, is provided in the controller 204 and in the remote site transmitter 208, respectively, for establishing the proper critical timing. An earth-orbiting vessel 201 transmits precision timing reference signals to each respective antenna 203, 207, 209 and 211 for establishing absolute timing for the controller 204 and each remote site transmitter 208, 210, etc. The controller 204 then calculates a "launch" time which is a predetermined, exact future time in which the message signal 202 (now buffered) is to be sent to a transmitting end of remote site transmitter 208, 210. The launch time takes into account a predetermined propagation delay of the interconnect links 224, 226. The launch time is next combined with the message signal 202 (message bundle) and sent to the remote site transmitters 208, 210 via the interconnect links 224, 226 of a distribution network 205.
Antenna 207 in combination with the receiver 221 receives a timing reference signal 215 which the remote site transmitter 208 uses as an exact, or absolute, timing reference. When the message bundle is received by the remote site transmitter 208 the launch time is removed therefrom and compared to the current absolute time provided by the timing reference signal 215. When these times are identical the remote site transmitter 208 (and 210, etc.) re-transmits via an antenna 212 the message signal 202 to the appropriate coverage area such that transmission times amongst the several remote site transmitters are substantially identical. The timing signals 213, 215, and 217 are typically phase-synchronized to within 100 nanoseconds. However, the phase difference between Phase-Locked-Loops (PLLs) resident at the several remote site transmitters may add additional phase differences resulting in further undesirable timing differences.
The phase difference considerations between the several remote site transmitters can be better appreciated by reference to FIGS. 3 and 4. FIG. 3 provides a more detailed block diagram of the controller 204 wherein a frequency reference signal 312 and a timing reference signal 314, are generated by the Global Positioning System (GPS) receiver 219 in response to the timing reference signal 213. The frequency reference signal 312 is input into a Phase Lock Loop (PLL) circuit 302 for generating a clock in signal 316 which is further input into a clock generator circuit 304 along with the timing reference signal 314 for updating an absolute time clock therein and synchronizing clocks within the several remote site transmitters. Clock generator 304 also sends a master synchronization input signal 320 to a combiner 306, which is used along with the timing reference signal 314 to produce a time stamp for combining with the message signal 202. Clock in signal 316 is divided by a divide-by-n circuit 310 which is input into the combiner 306 to establish a data rate at which the message bundle is transferred onto the distribution network 205. A 9.6 Khertz data rate could be established, for example, by switching the clock in signal 316 at 3.072 Mhertz and using a divide-by-n circuit wherein n is equal to 320.
FIG. 4 is a block diagram representation of the remote site transmitter 208 having a GPS receiver 221 for receiving the timing reference signal 215 (FIG. 2) via the antenna 207 for generating a frequency reference signal 405 and a timing reference signal 407. The frequency reference signal 405 is input into a PLL 425 for generating a site frequency reference signal, switching, for example, at 16.8 Mhertz, for the remote site transmitter 208. By action of a PLL 427, the frequency reference signal 405 is also used for generating a clock in signal 409. A clock generator 403 receives the clock in signal 409 to produce a master sync. in signal 411, a convert clock signal 413, and a receive clock signal 415 which signals are used to provide synchronizing and clock data to signal processing hardware including a processor 417 and D/A (digital to analog) converter 419.
The message bundle 224 is received by the remote site transmitter 208 at the processor 417, wherein the message bundle 224 is separated into the digital message data representing the message signal, and into launch time data. The digital message data is then placed into a data buffer 401 and held until the current time of day, provided by manipulation of the timing reference signal 407, matches exactly with the launch time provided in the message bundle. When this condition exists, the message data is passed to the D/A converter 419 to reconstruct the original message signal. The reconstructed message signal is then sent, via a transmitter 421, to the coverage area defined by the location of the remote site transmitter 208.
The accuracy in the time-of-arrival of the time-mark is not perfect. Some time-mark receivers average their time-mark solutions against their own internal clock. Time-mark receivers which output their time-solution directly without any time averaging or other time buffering present time-marks having the greatest amount of inaccuracies. The time-mark output from a time-mark receiver usually has a characteristic jitter due to the finest increment of time resolution, typically 100 nanoseconds. The time-mark solution will also typically have a time bias unique to the particular time sources which the receiver is solving from at a particular time (for GPS, this would be the particular choice of satellites). As the time-sources are handed off or changed, the unique time bias will likely change. This can appear as a step change in the timing from the time-mark receiver.
Additionally, the time solution will be subject to carrier signal-to-noise limitations and could temporarily jump to a substantial offset from Universal Coordinated Time (UTC) for one or several time solution periods. With signal-to-noise recovery or a change to an alternate time-source (for example, an alternate satellite), the timing solution could come back to the more typical offset. The transmitter, desirably, should operate off of the non-averaged time-mark solutions, which typically may have an uncertainty in timing of plus/minus one microsecond with occasional random larger errors. In a case of a GPS time-mark receiver failure, the time-mark may be output repetitively at a substantial offset from the correctly aligned time position.
Thus, what is needed is a simulcast transmission system with a remote site transmitter having a free wheeling capability and being responsive to commands for advancing or retarding transmitter timing via a digital phase locked loop without loss or corruption of modulation data.